✔ 最佳答案
Please see the deductions below:
圖片參考:
http://i117.photobucket.com/albums/o61/billy_hywung/Maths/Gensol.jpg
Note: The product to sum formula of sin A sin B should be:
sin Asin B = 0.5[cos (A-B) - cos (A+B)]
Hence sin 6xsin x = 0.5[cos (6x-x) - cos (x+6x)] = 0.5 (cos 5x - cos 7x) BUT NOT 0.5(cos 7x - cos 5x)